FYI: Celsius or Fahrenheit: which should we use?

Pick a number, subtract 32, divide by nine then multiply by five. (Don't do anything with your age.) You've just converted a Fahrenheit temperature into Celsius.

Both the Fahrenheit (proposed by Gdansk-born Daniel Gabriel Fahrenheit in 1724) and the Celsius (created by Swedish astronomer Anders Celsius in 1742) scales - the two most common measures of temperature - are based on the properties of water. Since water freezes and boils at temperatures that are easy to generate, it was the obvious substance on which to base a scale.

According to the man himself, Fahrenheit began devising his scale by determining three fixed points.

Zero was found by creating a frigorific (meaning a set temperature will always be reached) mixture of ice, water and ammonium chloride. He then put an alcohol or mercury thermometer into the mixture and let the liquid in the thermometer descend to its lowest point.

The second point was the 32nd degree, found by mixing just ice and water. His third point, the 96th degree, was the level of the liquid in the thermometer when held in the mouth or under the armpit.

Why 32 and 96? Before Fahrenheit began tinkering with his buckets of ice and water, Danish astronomer Ole Romer had already devised a scale. Romer had decided the boiling point of water should be 60 degrees, presumably because an hour has 60 minutes and a minute 60 seconds. Romer's zero was arbitrary, the main consideration apparently being that it should be colder than it ever got in Denmark. Romer's water froze at 71/2 degrees and his body temperature measured 221/2 degrees. Fahrenheit adopted Romer's scale but did away with the fractions, by multiplying by four, giving him 30 for the freezing point and 90 for body temperature. Then, for reasons nobody seems to understand, he multiplied his numbers by 16/15. His measurements were not entirely accurate and some time after his death in 1736 his scale was recalibrated slightly.